As known, data are transmitted in TDM (Time Division Multiplexing) transport networks in time windows or sets of slots (called “time slots”) whose length is fixed, each of which contains one or more circuit data flows defined by the implemented protocol. Networks of this kind include, for example, SDH (Synchronous Digital Hierarchy) or SONET (Synchronous Optical Network) networks. The term “slot” therefore means “time interval” in this case.
In WDM (Wavelength Division Multiplexing) networks, data are transmitted on different wavelengths λi, which replace the multiplexing time slots and consequently may be called “λ-slots”. Similarly, SDM (Space Division Multiplexing) or FDM (Frequency Division Multiplexing) networks can be implemented, in which case the concept of slot is associated to intervals of space or frequency. Specifically, FDM technology was implemented in the past before TDM technology was developed.
The invention refers to all types of networks based on the concept of slot described above. Reference will be made in the description that follows to TDM (SDH and SONET) and WDM networks considering their current popularity.
From a technical point of view, time slots in SDH and SONET networks and λ-slots in WDM networks can be considered equivalent. They are both “containers” in which data are transmitted, regardless of the actual physical implementation of the network.
Each transport network, SDH, SONET or WDM, can comprise various sub-networks, e.g. ring networks or meshed sub-networks, all employing the same transmission protocol.
When a circuit data flow set to be sold to a customer is created and established for operation, the slots forming the sub-network (time slots for SDH or SONET, λ-slots for WDM) are allocated to each circuit according to the situation of the network at the time; the situation is randomly determined by the incoming requests (according to the typical criteria of the network operator). In general, the aim is to optimise the total occupied band to minimise network implementation investments. Calculation algorithms have been developed for this purpose to optimise sub-network load and reduce band occupation. The traffic demand to be carried by the network must be entirely defined beforehand to work these theoretical algorithms, which in practice is impossible.
For example, a routing optimisation algorithm for SONET/SDH ring sub-networks is described in the publication HU-GON KIM, DONG-WAN TCHA, Optimal Load Balancing on Sonet Bidirectional Rings, Dankook University, July 1996.
The algorithm described in this publication is very useful for calculating optimal distribution of flows given a known demand, e.g. for programming a new ring sub-network. The biggest problem, on the other hand, to be faced when optimising a sub-network which is already operational in terms of occupied band, is how to change circuit configuration to implement the optimised configuration without disrupting the service, with minimum effects perceived by customers.
Flows are often untidy and irregular in a sub-network that has been working for some time due to flow allocation changes which inevitably occur in the course of time. For example, the allocated flows are released and “gaps” in the slot allocation map are formed when the resources allocated to a customer are cleared following cancellation of contracts. These gaps are often employed for smaller flows and even smaller gaps are created, which are even more difficult to use in the future. This effect in the middle term progressively worsens the ratio between the band effectively occupied by data in the sub-network and the total available band of sub-network, making the configuration of new flows in the sub-network effectively impossible.
Procedures for the temporary or definitive re-allocation of time slots in a TDM network are also known, e.g. the procedure illustrated in document WO 97/36402.
Despite referring to a switched network, this patent describes a procedure for shifting one or more time slots from a primary node to a temporary node and then re-allocating the time slots back to the primary node. This procedure is used to deal with the unexpected request for resources on the primary node by one or more customers and is used to temporarily allocate some time slots to an adjacent secondary node. The document shows how it is possible to work on time slot allocation, by shifting and exchanging them, without incurring excessive disruption for customers.
The object of this invention is to propose a procedure for dealing with the problem of how to optimise the band occupied by a plurality of circuit data flows in a transport sub-network while keeping the network running and causing the least possible disruption to customers for each single flow.
This and other objectives are obtained by means of an optimisation procedure and corresponding management device as illustrated in the annexed claims.